This invention relates to novel procedures for including one or more materials in homogeneous rubber-based formulations, e.g. natural and/or butyl rubber formulations.
Since the invention is particularly useful in large-scale or industrial processes for incorporating a rubber crosslinking agent in rubber-based adhesive formulations, the invention will be described with particularity by reference thereto.
My copending applications Ser. Nos. 823,222 filed Jan. 28, 1986 and No. 843,943 filed March 25, 1986 relate to protective coating systems for metal surfaces, e.g. tubular articles such as metal pipes and the like, which systems comprise: (1) a rubber-based primer coating mixture; and (2) an adhesive tape comprising a backing material carrying a partially crosslinked rubber-based adhesive coating, the system further including a crosslinking agent and a crosslinking activator adapted to provide an in situ additional crosslinking of the system after the primer coating and tape are applied.
In the commercial manufacture of the adhesive coating in the aforementioned systems, it is customary to admix the rubber components, tackifier, and other ingredients in an internal mixer, typically a Banbury to provide a homogeneous adhesive mix.
However, it is not possible to include in this mixing procedure the crosslinking agent necessary for the in situ crosslinking subsequent to applying the primer coating and adhesive tape to the metal article to be protected. Were the crosslinking agent to be so included in the Banbury mix, the heat generated in the mixing operation would cause further crosslinking of the rubber to occur, thereby minimizing or precluding the desired subsequent incipient crosslinking.
Consequently, in the manufacture of adhesive formulations of the foregoing description it is necessary to incorporate the crosslinking agent downstream from the Banbury in a subsequent mixing step.
The typical procedure one skilled in the rubber art would employ to do so would be to transport the rubber mix from the Banbury (hereinafter referred to as the "premix") to a rubber mill where the crosslinking agent is then incorporated. During this milling step, the mix is necessarily maintained at elevated temperatures, as is understood in the art. Following admixture of the crosslinking agent, the heated mix is transported to a station for coating on the backing material. Typically, this is accomplished by calendering, after which the resulting adhesive tape is cooled and wound.
A particularly preferred material for providing the adhesive backing is a polyolefin such as polyethylene. This material is customarily provided to the calendering step in molten form, and preparation of the adhesive tape is thus effected with a three-roll calender, as will be described with more particularity hereinafter.
From the foregoing description it will be seen that the milling step is accomplished while the mix is in a heated state and this elevated temperature is necessarily maintained thereafter through the calendering operation for preparing the tape. If the calendering is delayed and/or the milled mix need be stored prior to calendering, a warming mill may be required to heat the mix for calendering.
Apart from such cost factors as manpower, equipment, space and energy consumption, the period in which the mix is heated (subsequent to milling and prior to cooling after calendering) inherently permits crosslinking induced by the presence of the crosslinker in the mix. This (unwanted) crosslinking in turn expends the crosslinking agent, thereby materially reducing the possibility for the desired in situ crosslinking.
In view of the foregoing, there is a clear need for improved manufacturing procedures for incorporating the crosslinker and thereafter rapidly coating the mix on the backing material and then cooling to a temperature where crosslinking prior to use is precluded.
The present invention provides a simple and elegant solution to this problem.